A brushless direct current (DC) motor generally includes a rotor having a permanent magnet, and a stator having star-connected multiphase coils. In the brushless DC motor, the coils are excited by controlling current supplied to the coils and the rotor is rotated and driven relative to the stator. The brushless DC motor generally includes a sensor such as a Hall device, an optical encoder or the like in order to detect a rotational position of the rotor and provides a proper torque to the rotor by switching current supplied to the multiphase coils.
There has been proposed a sensorless motor in which a rotational position of a rotor is detected without using a sensor such as a Hall device or the like in order to make the motor smaller. The proposed sensorless motor obtains positional information by detecting an induced voltage generated in a coil by measuring a potential of a central point of a wiring of the motor, for example. However, it is impossible to know the rotational position of the sensorless motor when it is stopped since the positional information of the sensorless motor is obtained from the induced voltage generated during the rotation of the rotor. If the motor starts with the rotational position unknown, the rotor may be rotated reverse to a desired rotation direction. However, for example, in a case where a spindle motor of a hard disk is rotated, it is preferable to minimize reverse rotation of the spindle motor and, accordingly, there is a need to properly detect rotational position of the rotor when starting the motor.